toJSON: report error position for fancier output

Given flake:

```nix
{ description = "nix json error provenance";
  inputs = {};
  outputs = { self }: {
    jsonFunction = _: "function";
    json = builtins.toJSON (_: "function");
  };
}

```
- Before:

```console
❯ nix eval --json .#jsonFunction
error: cannot convert a function to JSON
```

- After:

```console
❯ nix eval --json .#jsonFunction
error: cannot convert a function to JSON

       at /nix/store/b7imf1c2j4jnkg3ys7fsfbj02s5j0i4f-source/testflake/flake.nix:4:5:

            3|   outputs = { self }: {
            4|     jsonFunction = _: "function";
             |     ^
            5|     json = builtins.toJSON (_: "function");
```

1 files changed, 15 insertions, 9 deletions

diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index bfea24d40..7c28754f5 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -10,11 +10,11 @@
 namespace nix {
 
 void printValueAsJSON(EvalState & state, bool strict,
-    Value & v, JSONPlaceholder & out, PathSet & context)
+    Value & v, const Pos & pos, JSONPlaceholder & out, PathSet & context)
 {
     checkInterrupt();
 
-    if (strict) state.forceValue(v);
+    if (strict) state.forceValue(v, pos);
 
     switch (v.type()) {
 
@@ -54,10 +54,10 @@ void printValueAsJSON(EvalState & state, bool strict,
                 for (auto & j : names) {
                     Attr & a(*v.attrs->find(state.symbols.create(j)));
                     auto placeholder(obj.placeholder(j));
-                    printValueAsJSON(state, strict, *a.value, placeholder, context);
+                    printValueAsJSON(state, strict, *a.value, *a.pos, placeholder, context);
                 }
             } else
-                printValueAsJSON(state, strict, *i->value, out, context);
+                printValueAsJSON(state, strict, *i->value, *i->pos, out, context);
             break;
         }
 
@@ -65,7 +65,7 @@ void printValueAsJSON(EvalState & state, bool strict,
             auto list(out.list());
             for (unsigned int n = 0; n < v.listSize(); ++n) {
                 auto placeholder(list.placeholder());
-                printValueAsJSON(state, strict, *v.listElems()[n], placeholder, context);
+                printValueAsJSON(state, strict, *v.listElems()[n], noPos, placeholder, context);
             }
             break;
         }
@@ -79,18 +79,24 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
 
         case nThunk:
-            throw TypeError("cannot convert %1% to JSON", showType(v));
+            throw TypeError({
+                .msg = hintfmt("cannot convert %1% to JSON", showType(v)),
+                .errPos = pos
+            });
 
         case nFunction:
-            throw TypeError("cannot convert %1% to JSON", showType(v));
+            throw TypeError({
+                .msg = hintfmt("cannot convert %1% to JSON", showType(v)),
+                .errPos = pos
+            });
     }
 }
 
 void printValueAsJSON(EvalState & state, bool strict,
-    Value & v, std::ostream & str, PathSet & context)
+    Value & v, const Pos & pos, std::ostream & str, PathSet & context)
 {
     JSONPlaceholder out(str);
-    printValueAsJSON(state, strict, v, out, context);
+    printValueAsJSON(state, strict, v, pos, out, context);
 }
 
 void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,